This invention relates to hydraulic systems. More particularly, this invention relates to hydraulic systems having more than one pump which can function alternately as one system or two systems.
Prudent design standards for contemporary commercial aircraft dictate that power redundancy be provided to flight control surfaces. Typically, ailerons, elevators, flaps, rudders, slats, and spoilers are moveable surfaces which may be hydraulically controlled as well as brakes, landing gear and steering. Redundancy may be supplied, in part, by multiple hydraulic systems, usually three in number. Control surfaces are further divided into those that move up and down and those that move in one direction only as in the latter case, symmetry may be essential and corresponding surfaces may have to move together on a single hydraulic system. However, assuming multiple spoilers on each wing, the outboard pair of spoilers on each wing may move on one hydraulic system while the center spoilers on a second system and the inboard spoilers on a third system. The rationale, of course, being that if an aircraft lost the No. 3 hydraulic system, all it would lose was the inborad set of spoilers on each wing, leaving the center pairs and outboard pairs active. Collection of control systems or other systems to be driven by a specific hydraulic system must take into consideration proximity of one system to another to accommodate major damage to all systems in a particular area and still maintain sufficient control surfaces to fly the airplane. When an aircraft has three or more engines, the obvious choice is to drive the three independent hydraulic systems from three separate engines. The three independent hydraulic systems become more vulnerable in a twin-engine configuration if two different systems are driven from a common engine as they both would be lost by failure of that particular engine. One alternative to provide redundancy is shown in U.S. Pat. No. 3,138,002 by Ernst et al which teaches three independent servo systems of equal capacity and if one fails, the pilot simply shuts that system off and selects a second system and still has a remaining backup. It is important to note that this system involves a positive shutoff valve to each system where total hydraulic power is cut off to the control surfaces. Shutoff valves in any control surface system are generally to be avoided as they will eventually be shut off whether inadvertently or otherwise when they need not be shut off. More interesting, is the teaching of this reference wherein one pump is driven by engine No. 1, the second pump by engine No. 2, and the third pump driven by both engines through a differential gear box. This redundancy, of course, is desirable, however, near impossible to achieve in modern aircraft. The rotating shafts must penetrate the fuselage. Further, if possible, it would not be weight efficient. The instant invention accomplishes this feature, and more, in an entirely different way.
Another alternative is to drive the third system with an electric motor which may be powered from either engine. However, this alternative sacrifices space and weight and still requires judicious location of wires to minimize vulnerability.